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Photocatalytic degradation of ketorolac tromethamine (KTC) drug in aqueous phase

using prepared Ag-doped ZnO microplates

Amandeep Kaur1, Alex O Ibhadon2 and Sushil Kumar Kansal1* 1Dr. S.S. B. University Institute of Chemical Engineering and Technology, Panjab

University, Chandigarh 2Department of Chemical Engineering, School of Engineering, University of Hull,

Cottingham Road, Hull, United Kingdom HU6 7RX 

Abstract

In this study, Ag-doped ZnO microplates were prepared via precipitation technique

and further characterized by FESEM, EDS, XRD, FTIR, TGA, XPS, UV-DRS and RT-PL

techniques. The outcomes indicated that Ag+ ions were well incorporated into ZnO lattice

leading to the absorption of ZnO in visible region as well as effective charge separation. The

photocatalytic experiments showed that Ag-doped ZnO microplates show higher catalytic

activity (91%) than bare ZnO (71%) for the degradation of KTC drug under solar

illumination. The photocatalytic degradation of KTC drug over Ag doped ZnO microplates

obeyed pseudo first-order kinetics model. Also, the role of active species was examined by

the addition of several scavengers in the photocatalytic degradation system. The results

indicated that h+, •OHs, 1O2 and •OH were considered as prime reactive species in

photocatalytic degradation process.

Keywords: Ag-doped ZnO, Strong adsorption, Ketorolac tromethamine, Solar light,

Photocatalysis

Highlights

Ag-doped ZnO photocatalyst have been synthesized using precipitation method

Ag-doped photocatalyst showed excellent photocatalytic activity for the degradation

of ketorolac tromethamine drug

The improved photoactivity is due to the effective charge separation

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*Corresponding authors

E-mail Address: sushilkk1@yahoo.co.in; sushilkk1@pu.ac.in

Ph: +91-172-2534920(O), 09876581564 (Sushil. K. Kansal)

1. Introduction

In recent years, researchers have put significant efforts in developing methods to

utilize solar energy more effectively in order to provide solution to one of the world’s major

problem-energy crisis. One promising approach is to synthesize highly active photocatalysts

to promote water splitting, photocatalytic treatment of environment pollutants, or both under

solar illumination [1-3]. Among various materials, metal oxide based nanostructures have

gained considerable attention owing to their high surface to volume ratio, adjustable band gap

and high stability [4-6]. ZnO has been regarded as a promising catalyst because of its low

cost, non-toxic nature and high electron stability [7-9]. But, it can only be activated under UV

light because of ample band gap (i.e. 3.2 eV), and more electron-hole recombination, thereby

its photocatalytic efficiency is still remained at slow rate because of limited adsorption of

solar light [10]. Therefore, recent research has been focused to design ZnO based

photocatalysts with broad wavelength for light absorption that utilize full solar spectrum [11].

In order to achieve broad wavelength spectrum for ZnO, two common approaches are

used - doping with metals (Ag, Au, Pt, etc.), non-metals (carbon, nitrogen, sulphur, etc.) and

composites with lower band gap semiconductors such as CuO, CdS, Fe2O3, CdSe, etc. [12-

16]. Among them, band-gap modification of ZnO through metal doping is considered as one

of the most effective approach to broaden the absorption of light into visible region by

suppressing electron-hole recombination. It was confirmed that doping with small

concentration increased the life span of photogenerated charge carriers and thereby improved

photocatalytic degradation efficiency [17, 18]. Recent studies have shown that silver is the

best doping element because of its larger ionic size, least orbital energy and high solubility

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[19]. It should be noted that not all transition metals give a positive response. In some cases,

transition metals reduce the photocatalytic degradation efficiency because of increase in

electron-hole recombination.

Herein, we report a precipitation method to prepare Ag-doped ZnO photocatalyst. The

as-prepared materials were further characterized using many techniques to examine their

compositional, morphological, optical and luminescent behaviour. The activity of the

photocatalyst was studied for the degradation of a biorecalcitrant organic pollutant, ketorolac

tromethamine (KTC) under solar irradiation. Also, the impact of pH and catalyst loading on

degradation efficiency was examined.

2. Experimental section

2.1. Materials and methods

Zinc acetate dehydrate (≥ 98% purity), silver nitrate (≥ 99% purity), sodium

hydroxide (> 97% purity), sodium azide (≥ 99.5% purity), sodium chloride (≥ 99% purity)

and potassium iodide (≥ 99% purity) were procured from Merck, India. Isopropanol (99%

purity) was obtained from SD Fine Chem Limited, India. Ketorolac tromethamine (KTC) was

provided by Saurav Chemicals Limited, Derabassi, India. Double distilled water (DDW) was

utilized to brew all stock solutions and reagents were used as received. 0.1 N solutions of HCl

and NaOH were added to regulate the pH of KTC solution with Mettler Toledo pH-meter.

Ag-doped ZnO photocatalyst was synthesized using similar precipitation method as

reported earlier [20]. To synthesize Ag-doped ZnO, firstly, 0.05 moles of zinc acetate

dihydrate were mixed in 50 mL double distilled water. Subsequently, 1M NaOH aqueous

solution was added dropwise to achieve the pH of the suspension to about pH 12. After that,

0.025 % of AgNO3 was dissolved in ethanol-water mixture (1:1) and then added dropwise

into above solution and stirred for 16 hours. The obtained precipitates was washed thoroughly

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with ethanol and then with DDW, filtered and finally dried in an oven at 80 °C for overnight.

Pure ZnO was also obtained using same procedure except adding up AgNO3 salt.

2.2. Characterization

The morphology of prepared catalyst was examined using field emission scanning

electron microscope (Hitachi-8010). The structure and crystallinity of synthesized samples

was examined on powder X-ray diffraction (PXRD) instrument. The scan analysis was

operated within 2θ range of 10-80° by Cu kα radiation. Fourier transform infrared (FTIR)

spectra of samples were obtained from Thermofisher Nicolet iS50 spectrometer in the range

of 400-4000 cm-1. TGA analysis was conducted on Perkin Elmer STA 6000 instrument using

nitrogen gas with a heating rate of 10 °C per minute. The chemical composition and

electronic states of samples were scrutinized on X-ray photoelectron spectroscopy (XPS)

with monochromated Mg Kα X-ray (hν = 1253.6 eV) radiation. UV-vis diffuse reflectance

(DRS) of samples was carried out on UV-vis (Shimadzu, UV-2600) spectrophotometer using

BaSO4 as reference. Room temperature photoluminescence (RT-PL) spectra of samples were

obtained using fluorescence spectrophotometer (Hitachi F-7000) at an excitation wavelength

of 340 nm.

2.3. Photocatalytic experiments

The photocatalytic activity of Ag doped ZnO was tested by degrading the KTC under

solar light irradiation (65-70 K lux, recorded on CHY 332 light meter). In a photocatalytic

experiment, 0.25 g of as-synthesized photocatalyst was dispersed into 100 mL of aqueous

solution of KTC drug. Adsorption-desorption equilibrium is accomplished in 30 minutes in

dark prior to light illumination to initiate the degradation reaction. Then, 2 mL aliquot was

extracted at specific time periods and filtered through 0.45 μm Chromafil syringe filter. The

absorbance of filtrate was measured using UV-vis Systronics-2202 spectrophotometer and

degradation efficiency was computed as:

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Degradation efficiency = [1-(C/C0)] × 100 (1)

where C0 is the initial KTC concentration and C is KTC concentration after irradiating with

solar light at a certain time.

3. Results and discussion

3.1. Characterization of Ag-doped ZnO powder

The electrode morphologies of Ag doped ZnO was observed using scanning electron

microscope (FESEM) and typical electron micrographs are shown in Fig. 1. As seen from

Fig. 1 (a-d), the prepared sample consists of 2-D plate like morphology and grown in high

density. Fig. 1(b) and (d) exhibits the high resolution images which verified that microplates

are formed by accretion of hundreds of nanoparticles. The typical thickness of plates is 40-50

nm and sizes are in the range of micrometers. EDS spectrum (Fig. 1 (e)) of Ag doped ZnO

identifies the existence of Ag in synthesized sample. XRD pattern of as-prepared ZnO and

Ag-doped ZnO are shown in Fig. 1 (f). In case of ZnO, all the diffraction peaks are well

matched with JCPDS data card no. 36-1451. XRD studies reveal that ZnO and Ag doped

catalysts exhibits various well defined peaks of wurtzite hexagonal ZnO. In Fig. 1 (f), the

diffraction peaks observed at 2θ = 31.74°, 34.47°, 36.42°, 47.53°, 56.68°, 62.88°, 66.43°,

67.92°, 69.18°, 72.60° and 77.19° can be related to the crystal planes of (100), (002), (101),

(102), (110), (103), (200), (112), (201), (004) and (202), respectively [21,22]. The peak

corresponds to (101) is more intense than other peaks and shows very small shifts towards

decreasing 2θ value with Ag doping. The slight increase in intensities of all the peaks for Ag

doping (0.025%) indicates improved crystallinity of ZnO [23]. No diffraction peak allied to

Ag is detected that can be due to very small concentration of Ag+ ions in sample, which

confirms the replacement of Zn2+ by Ag+ ions into ZnO matrix. The crystallite sizes of the

prepared samples are calculated from Debye Scherer’s equation.

D = 0.9λ/β Cos θ (2)

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where λ is the wavelength of the X-ray light, β is the broadening of diffraction peak at

FWHM and θ is Bragg’s angle. The lattice constants ‘a’ and ‘c’ are estimated from following

equation:

The lattice constants for ZnO and doped ZnO are a = 3.239Å, c = 5.197Å and a = 3.251 Å, c=

5.201 Å, respectively. The crystallite size of pristine and doped ZnO is estimated as 38.45 nm

and 34.15 nm respectively. The values of lattice parameters are increased with the doping

which is due to the inclusion of Ag+ ions into ZnO lattice or substitution of Zn2+ with Ag+

ions because of larger difference in radius of Zn2+ (0.074 nm) and Ag+ (0.126 nm) ions [24].

(3)

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Fig.1. FESEM images (a), (c) at low resolution, (b), (d) at high resolution (e) EDS and (d)

XRD of Ag doped ZnO microplates

The as-prepared products were examined in terms of their atomic and molecular

vibration. The FTIR spectrum of Ag-doped ZnO sample was obtained in the range of

wavenumber 400-4000 cm-1 which followed the similar pattern as that of pristine ZnO and as

shown in Fig 2. The peak at 3373 cm-1 corresponded to the absorption of surface hydroxyl

groups [25, 26]. The broad band positioned at 560 cm-1 and 876 cm-1 ascribed to the

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stretching vibrations of Zn-O [27]. It was inferred that the shift in band position could be as a

result of introduction of Ag+ ions into ZnO matrix. From the TGA analysis, it was seen that

the total weight loss (%) for Ag-doped ZnO and ZnO microplates were found to be 6.4 % and

5.3% respectively, in the range of 0 to 800 0C. The initial weight loss for both samples up to

500 °C was due to the evaporation of physically and chemically adsorbed water molecules on

the surface of microplates [28-30]. But, the weight loss at higher temperature (500-800 °C)

was about 0.5% and 1.2% for ZnO and Ag doped ZnO respectively, which depicts thermal

stability of prepared samples.

Fig. 2. (a) FTIR spectra (b) TGA of ZnO and Ag-doped ZnO microplates

Surface composition of the silver doped ZnO microplates were investigated by X-ray

photo electron spectroscopy. Fig. 3 (a)-(e) displays the scan survey spectra of Ag doped ZnO

and all the peaks on the curve may be corresponded to Zn, Ag, O and C element, while C 1s

at 284.8 eV is because of adventitious hydrocarbon (Fig. 3 (d)) from instrument itself. Fig. 3

(a) exhibits the Zn 2p binding energy region. The Zn 2p3/2 and 2p½ spin orbital states for

sample is positioned at binding energy of 1023.0 eV and 1045.6 eV, respectively [31, 32]. As

observed from high resolution Ag 3d spectrum (Fig. 3 (b)), Ag-doped ZnO exhibited twice

peaks identified at 365.5 eV and 372.7 eV were corresponding to Ag 3d5/2 and Ag 3d3/2

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respectively [33, 34]. These values are well agreement with metallic silver values [35, 36]. In

O 1s spectrum (Fig. 3 (c)), peak obtained at binding energy 529.7 eV ascribed to the lattice

oxygen of ZnO. The peaks found in overall XPS spectrum (Fig. 3 (e)) were in accordance

with the earlier results [37].

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Fig. 3. XPS spectra of Ag-doped ZnO microplates for (a) Zn 2p; (b) Ag 3d; (c) O 1s; (d) C 1s

and (e) Full spectrum.

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UV-vis DRS experiment was carried out to examine the light absorbed by prepared

catalyst. The UV-vis spectra of the Ag-doped ZnO and ZnO catalysts are shown in inset of

Fig. 4 (a). The synthesized Ag doped ZnO sample showed absorption of light shifted to a

longer wavelength region as compare to bare ZnO. A classical Tauc method was further used

to estimate the energy band gap of ZnO and Ag doped ZnO samples according to given

equation [38]:

(αhν)n = A (hν-Ebg) (2)

where α, h, ν, Ebg and A are absorption coefficient, planck constant, light frequency, band gap

and constant respectively. Among them, n is calculated from the optical transmission of a

semiconductor. The value of n for Ag doped ZnO and ZnO are taken as 2, because of the

characteristic of direct band transition. Thus, the energy band gap of ZnO and Ag doped ZnO

can be computed from a plot of (αhν)2 versus hν, (Fig 4 (a)), and were found to be 3.18 eV

and 3.10 eV, respectively. Furthermore, the substitution of silver ions into Zn2+ sites showed

red shift in band gap absorption of ZnO microplates as reported in literature [39].

Optical properties of prepared catalysts were monitored using RT-PL at an excitation

wavelength of 340 nm and results were shown in Fig. 4 (b). The PL spectrum of ZnO

possesses major UV-emission peak at 392 nm is because of the extinction of excitons and

visible light luminescence bands centred at 427 nm and 467 nm was ascribed due to the high

density of surface defects, oxygen vacancies and the recombination of free charge carriers

[36]. The green emission band centred at 528 nm is because of electron-hole recombination

which occupies single ion oxygen vacancy [37]. The PL intensity of Ag-doped ZnO was

reduced in contrast to pristine ZnO. The PL intensity of ZnO was significantly quenched by

the substitution of Ag in ZnO matrix [40], and thus Ag-doped ZnO may show higher

photocatalytic activity than ZnO catalyst.

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Fig. 4. (a) plot of (αhν) versus energy (hν), (inset) UV-vis DRS spectra; (b) RT-PL spectra of

as-synthesized ZnO and Ag-doped ZnO microplates

3.2. Photocatalytic performance

In order to assess the photocatalytic activity of Ag-doped ZnO microplates, various

degradation experiments were performed to degrade the KTC drug under solar irradiation.

Ketorolac tromethamine (KTC) exhibits an absorption peak at λmax = 320 nm. Fig. 5 (a)

shows photodegradation of KTC over Ag-doped ZnO microplates under solar light. It is

observed that the absorbance of drug solution gets reduced during photocatalytic reaction.

About 91% of KTC drug solution was degraded within 110 minutes of solar irradiation,

higher than that of pristine ZnO (71%) under similar conditions. It can be examined that no

considerable degradation of drug solution occurred only under solar light. However, about

32% adsorption of drug was observed on the surface of Ag-doped ZnO as shown in Fig 5 (b).

The effect of initial drug pH on degradation efficiency with Ag-doped ZnO was

studied by varying the pH of drug solution (10 mg/L) from 5 to 11, keeping all other

conditions constant i.e. catalyst dose (0.25 g/L). The degradation efficiency of KTC

enhanced from 80% to 91%, as pH increases from 5 to 7 and achieved maximum degradation

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efficiency (91%) at pH 7. On further increasing the pH from 9 to 11, degradation efficiency

decreased from 90% to 88% (Fig 5 (c)). The photodegradation efficiency of catalyst rely on

the acessiblility of active sites, so it is required to optimize the catalyst dose for the

degradation of drug compound.

To optimize the catalyst dose for the degradation of KTC (10 mg/L), number of

experiments were performed by altering the catalyst loading from 0. 15 g/L to 0.5 g/L at pH

7. It was noticed with the increase in catalyst loading from 0.15 g/L to 0.25 g/L, the

degradation efficiency of drug increased. However, as the dose was increased from 0.25 g/L

to 0.5 g/L, photodegradation efficiency was decreased which could be due to the obstruction

in the scattering of sunlight in hazy suspension (Fig 5 (d)). The photocatalytic degradation of

ketorolac tromethamine using Ag doped ZnO has not yet been described in literature. But, Ag

doped ZnO used as photocatalyst for the degradation of other organic pollutants such as dyes

and phenols [41-43]. Yildirim et al. [44] studied the photocatalytic degradation of methylene

orange with Ag-doped zinc oxide nanoparticles. Authors concluded that improved

photocatalytic degradation of dye was obtained with Ag-doped ZnO as compared to pristine

ZnO. Complete degradation of MO dye was achieved in 90 minutes under UV light with

0.3% Ag doped ZnO. Udom et al. [45] investigated the effect of Ag concentration on removal

efficiency of methyl orange and results exhibited that about 99% of methyl orange (20 mg/L)

was achieved in 2 hours with 1.2% Ag-doped ZnO under UV irradiation. Another study

showed 99.5% degradation of methyl orange in 60 minutes using Ag/ZnO under simulated

solar light [46]. In the present work, the prepared catalyst exihibits similar results ~91%

photocatalytic activity for the degradation of KTC under solar irradiation. The reaction

kinetics of ketorolac tromethamine (10 mg/L, pH 7) degradation was examined with

synthesized Ag-doped ZnO (0.25 g/L), using Langmuir-Hinshelwood kinetic model [17, 26].

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According to which, ln C0/C = kt, where C0 is the inital concentration of drug and C is the

final concentration of drug after photocatalysis at different time (t) intervals, k is the apparant

rate constant and t is the degradation reaction time. Fig 5 (e) exhibits the plot of ln(C0/C) vs

time for the photocatalytic degradation of KTC. It obeyed pseudo first order kinetic model

with apparent reaction rate constant (k) of 0.02287 min-1.

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Fig. 5. (a) UV-vis absorbance spectra of KTC; (b) Comparison of photolysis, adsorption and

photocatalytic activity of bare ZnO and Ag-doped ZnO under solar light (catalyst dose 0.25

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g/L, drug concentration 10 mg/L, pH 7); effect of (c) pH of drug solution; (d) catalyst dose on

degradation efficiency and (e) Plot of lnC0/C vs time demonstrating photocatalytic

degradation of KTC drug kinetics.

3.3. Role of primary reactive species

To determine the contribution of reactive species, various scavengers (0.01 M) were

introduced into photodegradation system prior to catalyst addition. The scavengers such as

potassium iodide (KI) for h+ and •OHs, sodium azide (NaN3) for 1O2 and •OH, sodium

chloride (NaCl) for h+ and isopropanol (IPA) for •OH were employed in this study [47-50].

As shown in Fig. 6, with the addition of KI, the degradation efficiency of KTC declined to

great extent, depicting that h+ and •OHs plays important role in photocatalytic process. In

addition, major inhibition effect on degradation performance was witnessed when NaN3 was

employed to quench 1O2 and •OH that verifies the significant role of reactive species in the

photocatalytic degradation process. In addition, the photocatalytic degradation of KTC (91%)

decreased to 37% and 41% after NaCl and IPA was added respectively, indicating h+, •OHs,

•1O2 and •OH are the prime reactive species in photocatalytic degradation process.

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Fig. 6 (a) The extent of photocatalytic degradation and (b) degradation efficiency of the KTC

solution (10 mg/L) with and without different scavengers over Ag-doped ZnO microplates

3.4. Photocatalytic degradation mechanism

The illustrative mechanism of solar light active photocatalytic reaction on the surface

of Ag-doped ZnO has been shown in Fig. 7. When light incidents on the surface of ZnO,

electrons-holes were generated in conduction and valence band, respectively (equation 5).

Schottky barrier is build up at the interface of ZnO and ensuing an efficient transfer of

electrons from ZnO to the newly formed interface. In dopant catalyst, the excition of

electrons are effective even with lower photon energy. Maenwhile, the Ag dopant act as an

electrons acceptor that traps the excited electrons from conduction band ZnO (equation 6)

[50], inhibit the electron-hole recombination and thus enhancing photocatalytic efficiency

[51, 52]. The photogenerated e─ react with O2 to form O2•─ (equation 7). The photogenerated

holes can easily captured by ─OH as well as H2O to produce hydroxyl radicals (•OH) in

equation (equation 8). The overall photocatalytic mechanism (equation 9) is based on the

reaction between pollutant and generated active species (O2•─ and •OH). Ag dopant can

extend the absorption of light by enhancing the photoresponse of cation loaded ZnO in entire

spectrum of solar light. However, high dopant concentration increases recombination rate of

photoexcited charge carriers and thereby decreasing photodegradation efficiency.

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Fig. 7. Photocatalytic degradation mechanism over Ag-doped ZnO microplates under solar

irradiation

ZnO + hν ZnO (e─CB + h+

VB ) (5)

Ag Ag+ + e─CB (6)

e─CB + O2 O2

•─ (7)

ZnO (h+VB) + ─OH/H2O •OH + H+ (8)

•OH + drug compound CO2 + H2O + other simple molecules /degraded products (9)

4. Conclusions

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Ag-doped ZnO microplates have been synthesized using pH-mediated precipitation

technique and typified in terms of their structural, morphological and optical properties. The

as-synthesized Ag-doped ZnO microplates were employed as an potent photocatalyst and

about 91% degradation of KTC was achieved in 110 minutes under solar light. The Ag-doped

ZnO photocatalyst also exhibited better photodegradation efficiency as compared to pristine

ZnO (71%). The enhanced photocatalytic efficiency of prepared photocatalyst is due to the

formation of barrier in ZnO interface and Ag dopant. Meanwhile, Ag dopant acts as electron

acceptor and the electrons moved from conduction band of ZnO to new formed interface and

thereby decreases the e─ + h+ recombination.

Acknowledgement

The authors greatly acknowledge the TEQIP-II grant of Dr S. S. Bhatnagar UICET, Panjab

University, Chandigarh for funding.

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